We form methane hydrate by injecting methane gas into a brine‐saturated, coarse‐grained sample under hydrate‐stable thermodynamic conditions. Hydrate forms to a saturation of 11%, which is much lower than that predicted assuming three‐phase (gas‐hydrate‐brine) thermodynamic equilibrium (67%). During hydrate formation, there are temporary flow blockages. We interpret that a hydrate skin forms a physical barrier at the gas‐brine interface. The skin fails periodically when the pressure differential exceeds the skin strength. Once the skin is present, further hydrate formation is limited by the rate that methane can diffuse through the solid skin. This process produces distinct thermodynamic states on either side of the skin that allows gas to flow through the sample. This study illuminates how gas can be transported through the hydrate stability zone and thus provides a mechanism for the formation of concentrated hydrate deposits in sand reservoirs. It also illustrates that models that assume local equilibrium at the core‐scale and larger may not capture the fundamental behaviors of these gas flow and hydrate formation processes.

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We form methane hydrate in brine‐saturated, coarse‐grained samples, under hydrate‐stable conditions, by injecting methane vapor at various flow rates. Decreasing the flow rate results in higher hydrate saturation, lower brine saturation, a smaller affected volume, and larger average pressure differentials across the sample. We interpret that the longer execution times at lower flow rates allow for additional methane transport and hydrate formation at the hydrate‐brine interface. As a result, the hydrate skin is thicker at lower flow rates and thus is capable of sustaining larger pressure differentials. In several experiments, we stop brine flow and supply methane gas to the sample for an additional 800 hrs. During this period, hydrate continues to form, pressure differentials develop, and the bulk density changes within the affected volume. We interpret that there is gas present in the sample that is disconnected from the gas source. Hydrate forms around the disconnected gas due to methane transport through the skin that surrounds it, causing the internal gas pressure to decline and leading to inward collapse and net volume decrease. This lowers the brine pressure and creates a differential pressure across the sample that induces gas flow. This study indicates that lower gas flow rates through the hydrate stability zone can produce very high saturations of hydrate but require a larger differential pressure to sustain flow. Ultimately, this process is an alternative mechanism for sustained upward gas flow and hydrate formation far above the base of the hydrate stability zone.

We describe gas flow through the hydrate stability zone as a growing gas network with hydrate forming on its edges. The diffusion‐limited model successfully describes the dynamics of hydrate formation observed experimentally. We obtain an expression for the rate of hydrate growth in a region where gas and water coexist. Our modeled rate of hydrate formation is much lower than predicted with a kinetics‐based approach, and it more successfully simulates the hydrate formation rate observed in geologic systems. Current simulation approaches can readily incorporate our approach to describe hydrate formation in geological systems.

Methane is a primary component of the “ice” layers in icy bodies whose thermal transport properties and velocity‐density profiles are essential to understanding their unique geodynamic and physiochemical phenomena. We present experimental measurements of methane's thermal conductivity and compressional velocity to 25.1 and 45.1 GPa, respectively, at room temperature, and theoretical calculations of its equation of state, velocity, and heat capacity up to 100 GPa and 1200 K. Overall, these properties change smoothly with pressure and are generally unaffected by the imposed atomic structure; though we observe a discrete spike in conductivity near the I‐A phase boundary. We cross‐plot the thermal conductivity and compressional velocity with density for the primary “ice” constituents (methane, water, and ammonia) and find that methane and water are the upper and lower bounds, respectively, of conductivity and velocity in these systems. These physical properties provide critical insights that advance the modeling of thermo‐chemical structures and dynamics within icy bodies.